After graduating Hughes began working for Chamberlain, Carter & Hornblower where he met his future wife. In 1888, shortly after he was married, he became a partner in the firm, and the name was changed to Carter, Hughes & Cravath. Later the name was changed to Hughes, Hubbard & Reed. In 1891, Hughes left the practice of law to become a professor at Cornell Law School. In 1893, he returned to his old law firm in New York City to continue practicing until he ran for governor in 1906. He continued his association with Cornell as a special lecturer at the Law School from 1893 to 1895. He was also a special lecturer for New York University Law School, 1893–1900.

At that time, in addition to practicing law, Hughes taught at New York Law School with Woodrow Wilson, who would later defeat him for the Presidency. In 1905, he was appointed as counsel to the New York state legislative "Stevens Gas Commission", a committee investigating utility rates.[5] His uncovering of corruption led to lower gas rates in New York City. In 1905, he was appointed to the "Armstrong Insurance Commission" to investigate the insurance industry in New York as a special assistant to U.S. Attorney General.

As the Governor, Hughes produced important reform legislation in three areas: improvement of the machinery and processes of government; extension of the state's regulatory authority over businesses engaged in public services; and expansion of governmental police and welfare functions.[6] To counter political corruption, he secured campaign laws in 1906 and 1907 that limited political contributions by corporations and forced candidates to account for their receipts and expenses, legislation that was quickly copied in fifteen other states. He pushed the passage of the Moreland Act, which enabled the governor to oversee city and county officials as well as officials in semi-autonomous state bureaucracies. This allowed him to fire many corrupt officials. He also managed to have the powers of the state's Public Service Commissions increased and fought strenuously, if not completely successfully, to get their decisions exempted from judicial review.

When two bills were passed to reduce railroad fares, Hughes vetoed them on the grounds that the rates should be set by expert commissioners rather than by elected ones. His ideal was not government by the people but for the people. As Hughes put it, "you must have administration by administrative officers."[1]

Hughes, however, would be unsuccessful in achieving one of his main goals as governor: primary voting reform.[6] Hoping to achieve a compromise with the state's party bosses, Hughes rejected the option of a direct primary in which voters could choose between declared candidates and instead proposed a complicated system of nominations by party committees. The state's party bosses, however, rejected this compromise and the state legislature rejected the plan on three occasions in 1909 and 1910.

On social issues, Hughes strongly supported relatively limited social reforms. He endorsed the Page-Prentice Act of 1907, which set an eight-hour day and forty-eight-hour week for factory workers—but only for those under the age of sixteen. By employing the well-established legal distinction between ordinary and hazardous work, the governor also won legislative approval for a Dangerous Trades Act that barred young workers from thirty occupations. To enforce these and other regulations, in 1907 Hughes reorganized the Department of Labor and appointed a well-qualified commissioner. Two years later, the governor created a new bureau for immigrant issues in the Department of Labor and appointed reformer Frances Kellor to head it.

In his final year as the Governor, he had the state comptroller draw up an executive budget. This began a rationalization of state government and eventually it led to an enhancement of executive authority. He also signed the Worker's Compensation Act of 1910, which required a compulsory, employer-paid plan of compensation for workers injured in hazardous industries and a voluntary system for other workers; after the New York Court of Appeals ruled the law unconstitutional in 1911, a popular referendum was held that successfully made the law an amendment in the New York Constitution.

In 1908, Governor Hughes reviewed the clemency petition of Chester Gillette concerning the murder of Grace Brown. The governor denied the petition as well as an application for reprieve, and Gillette was electrocuted in March of that year.

When Hughes left office, a prominent journal remarked "One can distinctly see the coming of a New Statism ... [of which] Gov. Hughes has been a leading prophet and exponent". In 1926, Hughes was appointed by New York Governor Alfred E. Smith to be the chairman of a State Reorganization Commission through which Smith's plan to place the Governor as the head of a rationalized state government, was accomplished, bringing to realization what Hughes himself had envisioned.

In 1909, Hughes led an effort to incorporate Delta Upsilonfraternity. This was the first fraternity to incorporate, and he served as its first international president.

On April 25, 1910, President William H. Taft nominated Hughes for Associate Justice to fill the vacancy left by the death of Justice David J. Brewer. The Senate confirmed the nomination on May 2, 1910, and Hughes received his commission the same day. As an associate justice of the Supreme Court from 1910 to 1916, Hughes remained an advocate of regulation and authored decisions that weakened the legal foundations of laissez-faire capitalism. He also mastered a new set of issues regarding the Commerce Clause and, in a deliberately restrained manner, wrote constitutional decisions that expanded the regulatory powers of both the state and federal governments.

On April 15, 1915 in the case of Frank v. Mangum,[10] the Supreme Court decided (7-2) to deny an appeal made by Leo Frank's attorneys, and instead upheld the decision of lower courts to sustain the guilty verdict against Frank. Justice Hughes and Justice Oliver Wendell Holmes Jr. were the two dissenting votes.

Hughes resigned from the Supreme Court on June 10, 1916,[11] to be the Republican candidate for President in 1916. He is the last sitting Supreme Court justice to surrender his or her seat to run for elected office. He was also endorsed by the Progressive Party,[12] thanks to the support given to him from former President Theodore Roosevelt. Other Republican figures such as former President William Howard Taft endorsed Hughes and felt the accomplishments he made as Governor of New York would establish him as formidable progressive alternative to Wilson.[1] Many former leaders of the Progressive Party, however, endorsed Wilson because Hughes opposed the Adamson Act, the Sixteenth Amendment and diverted his focus away from progressive issues during the course of the campaign.[1] Hughes was defeated by Woodrow Wilson in a close election (separated by 23 electoral votes and 594,188 popular votes). The election hinged on California, where Wilson managed to win by 3,800 votes and its 13 electoral votes and thus was returned for a second term; Hughes had lost the endorsement of the California governor and Roosevelt's 1912 Progressive running mate Hiram Johnson when he failed to show up for an appointment with him.

Despite coming close to winning the presidency, Hughes did not seek the Republican nomination again in 1920. Hughes also advocated ways to prevent the return of President Wilson's expanded government control over important industries such as the nation's railroads,[1] which he felt would lead to the eventual destruction of individualism and political self-rule.[1] After Robert LaFollette's Progressive Party advocated the return of such regulations during the 1924 US Presidential election, Hughes shifted rightwards believing that the federal bureaucracy should now have limited powers over individual liberties and property rights and that common law should be strictly enforced.[1]

Hughes returned to government office in March 1921 as Secretary of State under President Harding. On November 11, 1921, Armistice Day (later changed to Veterans Day), the Washington Naval Conference for the limitation of naval armament among the Great Powers began. The major naval powers of Britain, France, Italy, Japan and the United States were in attendance as well as other nations with concerns about territories in the Pacific — Belgium, the Netherlands, Portugal and China.[13]

Hughes continued in office after Harding died and was succeeded by Coolidge, but resigned after Coolidge was elected to a full term. On June 30, 1922, he signed the Hughes–Peynado agreement that ended the United States's six-year occupation of Dominican Republic.[15]

Herbert Hoover, who had appointed Hughes's son as Solicitor General in 1929, appointed Hughes Chief Justice of the United States on February 3, 1930. Hughes was confirmed by the United States Senate on February 13, 1930, and received commission the same day, serving in this capacity until 1941. Hughes replaced former President William Howard Taft, a fellow Republican who had also lost a presidential election to Woodrow Wilson (in 1912) and who, in 1910, had appointed Hughes to his first tenure on the Supreme Court.

Hughes' appointment was opposed by progressive elements in both parties who felt that he was too friendly to big business. Idaho Republican William E. Borah said on the United States Senate floor that confirming Hughes would constitute "placing upon the Court as Chief Justice one whose views are known upon these vital and important questions and whose views, in my opinion, however sincerely entertained, are not which ought to be incorporated in and made a permanent part of our legal and economic system."[18] In addition to his politics, at 67, Hughes was the oldest man ever nominated as Chief Justice.[19] Nonetheless Hughes was confirmed as Chief Justice with a vote of 52 to 26.

Upon his return to the court, more progressives had joined the bench. Hughes seemed determined again to vote progressive and soon bring an end to the longstanding pro-business Lochner era.[1] During his early years as Chief Justice, however, the fear he had developed for an overblown bureaucracy during World War I undermined his optimism.[1] Showing his old progressive image, he upheld legislation protecting civil rights and civil liberties[1] and wrote the opinion for the Court in Near v. Minnesota283U.S.697 (1931), which held prior restraint against the press is unconstitutional. Concerning economic regulation, he was still willing to uphold legislation that supported "freedom of opportunity" for individuals on the one hand and the "police power" of the state on the other[1] but did not personally favor legislation that linked national economic planning and bureaucratic social welfare together.[1] At first resisting Roosevelt's New Deal and building a consensus of centrist members of the court, Hughes used his influence to limit the collectivist scope of Roosevelt's changes[1] and would often strike down New Deal legislation he felt was poorly drafted and did not clearly specify how they were constitutional.[20] By 1935, Hughes felt the court's four conservative Justices had disregarded common law and sought to curb their power.[1]

Hughes was often aligned with the court's three liberal Justices — Louis Brandeis, Harlan Fiske Stone, and Benjamin Cardozo — in finding some New Deal measures (such as the violation of the gold clauses in contracts and the confiscation of privately owned monetary gold) constitutional.[1] On one occasion, Hughes would side with the conservatives in striking down the New Deal's Agricultural Adjustment Act in the 1936 case United States v. Butler,[1] which held that the law was unconstitutional because its so-called tax policy was a coercive regulation rather than a tax measure and the federal government lacked authority to regulate agriculture.[1] But surprisingly he did not assign the majority opinion,[1] a practice usually required for court's most senior justice who agrees with the majority opinion,[21][22] and allowed Associate Justice Owen Roberts to speak for the entire majority in his own words.[1] It was accepted that he did not agree with the argument that the federal government lacked authority over agriculture and was going to write a separate opinion upholding the act's regulation policy while striking down the act's taxation policy on the grounds that it was a coercive regulation rather than a tax measure.[1] However, Roberts convinced Hughes that he would side with him and the three liberal justices in future cases pertaining to the nation's agriculture that involved the Constitution's General Welfare Clause if he agreed to join his opinion.[1]

By 1936, Hughes sensed the growing hostility in the court and could do little about it.[1] In the 1936 case Carter v. Carter Coal Company, Hughes took a middle ground for doctrinal and court-management reasons.[1] Writing his own opinion, he joined the three liberal justices in upholding the Bituminous Coal Conservation Act's marketing provision but sided with Roberts and the four conservatives in striking down the act's provision that regulated local labor.[1] By 1937, as the court leaned more in his favor, Hughes would renounce the position he took in the Carter case regarding local labor and ruled that the procedural methods that governed the Wagner Act's labor regulation provisions bore resemblance to the procedural methods which governed the railroad rates that the Interstate Commerce Commission was allowed to maintain in the 1914 Shreveport decision; he thus demonstrated that Congress could use its commerce power to regulate local industrial labor as well.[1]

In 1937, when Roosevelt attempted to pack the Court with six additional justices, Hughes worked behind the scenes to defeat the effort,[23][23] which failed in the Senate,[23] by rushing important New Deal legislation — such as Wagner Act and the Social Security Act — through the court and ensuring that the court's majority would uphold their constitutionality.[1] The month after Roosevelt's court-packing announcement, Roberts, who had joined the four conservative Justices in striking down important New Deal legislation, shocked the American public by siding with Hughes and the court's three liberal justices in striking down the court's ruling in the 1923 Adkins v. Children's Hospital case — which held that laws requiring minimum wage violated the Fifth Amendment's due process clause — and upholding the constitutionality of Washington state's minimum wage law in West Coast Hotel Co. v. Parrish. Because Roberts had previously sided with the four conservative justices and used the Adkins decision as the basis for striking down a similar minimum wage law the state of New York enforced in Morehead v. New York ex rel. Tipaldo,[24] it was widely perceived that he only agreed to uphold the constitutionality of minimum wage as a result of the pressure that was put on the Supreme Court by the court-packing plan.[25] However, Hughes and Roberts acknowledged that the Chief Justice had already convinced Roberts to change his method of voting months before Roosevelt announced his court-packing plan[20][26] and that the effort he put into defeating the plan played only a small significance in determining how the court's majority made their decisions in future cases pertaining to New Deal legislation.[26]

Following the overwhelming support that voters showed for the New Deal through Roosevelt's overwhelming re-election in November 1936,[20] Hughes was not able to persuade Roberts to base his votes on political maneuvering and to side with him in future cases regarding New Deal-related policies.[20][26] Roberts had voted to grant certiorari to hear the Parrish case before the election of 1936.[27] Oral arguments occurred on December 16 and 17, 1936, with counsel for Parrish specifically asking the court to reconsider its decision in Adkins v. Children's Hospital,[28] which had been the basis for striking down a New Yorkminimum wage law in Morehead v. New York ex rel. Tipaldo[24] in the late spring of 1936.[29]

Roberts indicated his desire to overturn Adkins immediately after oral arguments ended for the Parrish case on December 17, 1936.[29] The initial conference vote on December 19, 1936 was 4-4; with this even division on the Court, the holding of the Washington Supreme Court, finding the minimum wage statute constitutional, would stand.[30] The eight voting justices anticipated Justice Stone — absent due to illness — would be the fifth vote necessary for a majority opinion affirming the constitutionality of the minimum wage law.[30] As Hughes desired a clear and strong 5-4 affirmation of the Washington Supreme Court's judgment, rather than a 4-4 default affirmation, he convinced the other justices to wait until Stone's return before deciding and announcing the case.[30] In one of his notes from 1936, Hughes wrote that Roosevelt's re-election forced the court to depart from its "fortress in public opinion" and severely weakened its capability to base its rulings on personal or political beliefs.[20]

PresidentRoosevelt announced his court reform bill on February 5, 1937, the day of the first conference vote after Stone's February 1, 1937 return to the bench. Roosevelt later made his justifications for the bill to the public on March 9, 1937 during his ninth Fireside Chat. The Court's opinion in Parrish was not published until March 29, 1937, after Roosevelt's radio address. Hughes wrote in his autobiographical notes that Roosevelt's court reform proposal "had not the slightest effect on our [the court's] decision,"[25] but due to the delayed announcement of its decision the Court was characterized as retreating under fire.[25]

Hughes wrote 199 majority opinions in his time on the bench, from 1930 to 1941. "His opinions, in the view of one commentator, were concise and admirable, placing Hughes in the pantheon of great justices."[34] His "remarkable intellectual and social gifts...made him a superb leader and administrator. He had a photographic memory that few, if any, of his colleagues could match. Yet he was generous, kind, and forebearing in an institution where egos generally come in only one size: extra large!"[34]

A bust-length portrait of Hughes by the Swiss-born American portrait painter Adolfo Müller-Ury (1862–1947) is in the Michigan Historical Museum in Lansing, Michigan. It was accessioned by them in 1939–1940 but probably acquired earlier.

The New York City Bar Association has a room named after Charles Evans Hughes. Two portraits of him are hung in this room as well as one of his son, Charles Evans Hughes Jr.